Automatic marine cathodic protection system using galvanic anodes

Abstract

An automatic system uses sacrificial galvanic anodes to to provide a controlled and optimum amount of cathodic protection against galvanic corrosion on submerged metal parts. Intermittently pulsed control circuitry enables an electro-mechanical servo system to control a resistive element interposed between the sacrificial anodes and the electrically bonded underwater parts. In an active mode of operation a current is applied directly to the anodes to quickly establish the proper level of correction which is maintained during the passive mode. Incremental corrections are made over a period of time to provide stabilization of the protection and to conserve power. A visual indication of the amount of protection is available at all times. Circuitry and indicating devices are included which facilitate location and correction of potentially harmful stray currents and to prevent loss of sacrificial anodes to nearby marine structures.

We claim: 
     
       1. In a cathodic protection system protecting a metal part of a marine structure having an electrical ground, the part immersed in an electrolyte, the system comprising a sacrificial anode immersed in the electrolyte, an improvement comprising: a metal reference electrode continually immersed in the electrolyte;   a controller powered by an electrical power supply, the controller switching from an active mode having a first predetermined duration to a passive mode having a second predetermined duration, the controller powered in the active mode by the power supply and having, as inputs, electrical connections to the electrical ground and to the reference electrode, the controller having an electrical output when in the active mode, the controller having no electrical output when in the passive mode; and   a variable resistor electrically connected intermediate the sacrificial anode and the protected metal part; wherein the output of the controller controls the voltage between the reference electrode and the protected part by controlling the electrical resistance of the variable resistor.     
     
     
       2. The system of claim 1 wherein the electrical resistance of the variable resistor remains at the value last set during the active mode when the controller switches from the active mode to the passive mode. 
     
     
       3. The system of claim 1 further comprising a timer switching the controller between the active mode and the passive mode. 
     
     
       4. The system of claim 1 wherein the reference electrode comprises pure copper having less than 200 parts per million total impurities. 
     
     
       5. The system of claim 1 further comprising indicator means providing a visual display to an operator of the voltage between the reference electrode and the protected part. 
     
     
       6. The system of claim 1 wherein the electrical power supply is isolated from all other sources of electric power on the structure. 
     
     
       7. The system of claim 1 wherein the electrical power supply comprises a battery not connected to an electric power of the structure. 
     
     
       8. The system of claim 1 wherein the electrical power supply comprises a battery electrically connected to an electric power system of the structure, an oscillator and amplifier circuit electrically connected to the battery, and an isolation transformer electrically connected to the oscillator and amplifier circuit. 
     
     
       9. The system of claim 1 wherein the electrical power supply comprises an isolation transformer electrically connected intermediate the controller and an on-shore electric mains supply. 
     
     
       10. The system of claim 1 further comprising a diode string electrically connected intermediate a shore ground and the electrical ground of the marine structure, the diode string having a turn-on voltage exceeding the greatest voltage supplied by a single galvanic cell. 
     
     
       11. The system of claim 1 further comprising a diode string electrically connected intermediate a shore ground and the electrical ground of the marine structure, the diode string having a turn-on voltage lower than the voltage of an on-shore AC mains power supply. 
     
     
       12. The system of claim 1 comprising a plurality of protected parts, each part of the plurality of parts electrically bonded to the electrical ground. 
     
     
       13. In a cathodic protection system protecting a metal part of a marine structure having an electrical ground, the part immersed in an electrolyte, the system comprising a sacrificial anode immersed in the electrolyte, an improvement comprising a metal reference electrode continually immersed in the electrolyte;   a controller powered by an electrical power supply, the controller switchable between an active mode and a passive mode, the controller powered in the active mode by the power supply and having, as inputs, electrical connections to the electrical ground and to the reference electrode, the controller having an electrical output when in the active mode; and   a rheostat having a wiper contact driven by an electric motor, the motor controlled by the output of the controller; wherein the output of the controller controls the voltage between the reference electrode and the protected part by controlling a the electrical resistance of the rheostat.     
     
     
       14. The system of claim 13 wherein the controller comprises an operational amplifier having inverting and non-inverting inputs, the inverting input electrically connected to the reference electrode, the non-inverting input electrically connected to the electrical ground, the output of the controller controlling an electric motor moving the wiper contact. 
     
     
       15. The system of claim 14 further comprising a transistor bridge electrically connected intermediate the operational amplifier and the motor, the bridge having operatively connected to a first terminal thereof a positive DC voltage from the power supply, the bridge having operatively connected to a second terminal thereof a negative DC voltage from the electrical power supply. 
     
     
       16. A method of using an electronic controller having an output responsive to an input voltage to cathodically protect a metal portion of a marine structure, the metal portion immersed in an electrolyte, the method comprising the steps of: a) providing, as the input voltage, a voltage between the protected portion and a metal reference electrode continually immersed in the electrolyte;   b) changing, responsive to the output of the controller, the electrical resistance of a variable resistor electrically connected intermediate the protected portion and a sacrificial anode immersed in the electrolyte until either b1) the voltage between the protected portion and the reference electrode attains a predetermined value or   b2) a first predetermined time interval elapses;     c) holding the resistance of the variable resistor constant for a second predetermined time interval, and   d) repeating steps a) through c).   
     
     
       17. The method of claim 16 further comprising a start-up procedure executed prior to step a) thereof, the startup procedure comprising the steps of i) causing electric current to flow from a supply thereof through the sacrificial anode, the electrolyte and the metal portion,   ii) measuring the voltage between the reference electrode and the protected part, and   iii) disconnecting the supply from the sacrificial anode when the voltage attains a predetermined hull potential value.   
     
     
       18. The method of claim 16 further comprising a start-up procedure executed prior to step a) thereof, the startup procedure comprising the steps of i) causing electric current to flow from a supply thereof through the sacrificial anode, the electrolyte and the metal portion,   ii) measuring the current between the reference electrode and the protected part, and   iii) disconnecting the supply from the sacrificial anode when the current attains an equilibrium value.   
     
     
       19. Apparatus for testing the integrity of the electrical bonding of a metal part to an electrical ground on a marine structure in contact with an electrolyte, the marine structure protected from corrosion by a cathodic protection system comprising an electrically connected sacrificial anode, the apparatus comprising: a millivoltmeter having an input connected to a metal reference electrode continually immersed in the electrolyte, the millivoltmeter having a second input connected to an electrical common point;   a bonding probe electrically connected intermediate the common point and the metal part;   a switch having an open state and a closed state, the switch connecting the common point to the electrical ground when in the closed state, the switch isolating the common point from the electrical ground when in the open state, whereby a lack of integrity of the bonding is indicated by a change of more than a predetermined value in the voltage measured by the millivoltmeter, the change occurring on switching the switch from the closed to the open state thereof.     
     
     
       20. Apparatus of claim 19 wherein the predetermined value is ten millivolts. 
     
     
       21. Apparatus of claim 19 wherein the millivoltmeter is connected to the reference electrode through an amplifier powered by an electrical power supply electrically isolated from all other sources of electric power on the structure.